Platform RFI mitigation

ABSTRACT

In some embodiments, SSC (e.g., discrete SSC) profiles with intentional and controlled gaps may be used to mitigate interference for platform radios. Targeted frequency gaps are placed in spectrum of spread clocks and clock-derived signals where they may otherwise result in problematic RFI to a platform radio.

TECHNICAL FIELD

The present invention relates generally to computing devices with radiocomponents and in particular, to computing devices with clock adjustmentfeatures for mitigating against detrimental RFI.

BACKGROUND

Wireless computing platforms (or computing platforms) may communicateusing one or more wireless communication channels. With today's wirelessplatforms it is not possible to completely avoid platform radiofrequency interference (RFI). Platform components typically includeclocks that, during operation, may generate harmonics that overlap withthe frequency range of at least one wireless (radio) channel. In someplatforms, the close proximity of the clocks and wireless transceiversmay introduce significant RFI with one or more wireless channels. Theeffect of the RFI may be to significantly reduce the data rate and/oroperating range of the wireless channel.

Some computing platforms use spread spectrum clock (SSC) modulation tominimize electromagnetic emissions and ease compliance with regionalelectromagnetic interference (EMI) regulations. For example, FIG. 1A isa graph showing the spectrum of a harmonic near a WiFi 2.5 GHz band ofan un-spread 100 MHz clock (e.g., used to generate a PCI Express clockon the platform). FIG. 1C is a plot showing the spectrum of the sameharmonic after 1% of conventional triangular center-spread SSCmodulation is applied. (FIG. 1B shows one cycle of a typical spreadspectrum clocking profile with 1% peak-to-peak amplitude and 30 KHzfrequency.) Also shown in FIG. 1C is a particular WiMax channel.

SSC implementations generally reduce the peak electromagnetic energy toattain EMI compliance by spreading energy across adjacent frequencies.As seen in FIG. 1C, the peak energy has been reduced, but the energy hasbeen spread into a WiMax channel, along with adjacent WiFi channels. So,while such SSC modulation can redress EMI issues, unfortunately, theenergy is often spread into sensitive areas used by platform radios toreceive information. Since platform radios are typically sensitive tonoise levels below EMI limits (typically 30 to 40 dB more sensitive),this behavior can seriously degrade radio performance.

Accordingly, a new approach for impeding wireless interference whilebeing able to maintain EMI compliance, if possible, is needed.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention are illustrated by way of example, and notby way of limitation, in the figures of the accompanying drawings inwhich like reference numerals refer to similar elements.

FIG. 1A is a graph showing platform RFI generated by a platform clockharmonic.

FIG. 1B is a graph showing a spread spectrum clock (SSC) profile to beapplied to the clock generating the harmonic of FIG. 1A.

FIG. 1C is a graph showing platform RFI generated by the clock of FIG.1A after it has been modulated using an SSC profile of FIG. 1B.

FIG. 2A is a graph showing an SSC profile for the clock of FIG. 1A togenerate a notched spectrum in accordance with some embodiments.

FIG. 2B is a graph showing the notched RFI spectrum resulting fromapplication of the SSC profile of FIG. 2A in accordance with someembodiments.

FIG. 3 is a diagram showing a computing platform with RFI mitigationusing SSC in accordance with some embodiments.

FIG. 4 is a flow diagram showing a routine for mitigating problematicRFI in the platform of FIG. 3 in accordance with some embodiments.

FIG. 5 is a diagram of another computing platform with SSC RFImitigation in accordance with some embodiments.

DETAILED DESCRIPTION

In some embodiments, SSC (e.g., discrete SSC) profiles with intentionaland controlled gaps may be used to mitigate interference for platformradios. Targeted frequency gaps are placed in spectrum of spread clocksand clock-derived signals where they may otherwise result in problematicRFI to a platform radio. The gaps are suitably aligned with wirelessplatform channels or other bands of interest to mitigate interferencewith platform radio operation. The gaps are controlled by design of thespread profile.

FIG. 2A is an example of a particular discrete SSC profile that may beused to spread out spectral energy in order to comply with EMIrequirements and at the same time, notch out energy for a platformchannel of interest. (In the depicted example, the notch locationcorresponds to the WiMax channel from FIG. 1C.) FIG. 2B is the resultingspectrum for the 2.5 GHz harmonic. It shows that an RFI notch is createdfor a WiMax channel.

In general, any SSC profile (e.g., triangular, sinusoidal, Hershey Kiss™(Lexmark), etc.) may be used, so long as it includes sufficientreductions, if not complete omissions, of frequency components ofinterest. For example, with the profile of FIG. 2A, use of frequenciesranging from about 99.8 to 100.3 MHz are omitted. This appears as avertically displaced gap, or at least fairly “steep”, steps in theprofile. To implement such profiles, discrete spread spectrum clocking(DSSC) may be preferred, if not required. (Note that as used herein,“vertically displaced” means to have a substantial vertical component,but exclusively vertical, i.e., rectangular shaped, gaps are notrequired. Ramps, e.g., steep ramps, may also be suitable, depending ondesign considerations. So, a trapezoidal shaped, rather than rectangularshaped, gap may be sufficient.) Persons of ordinary skill in the artshould be sufficiently familiar with DSSC technologies. As an example ofa DSSC implementation, reference is made to U.S. patent application Ser.No. 11/726,911 entitled: SPREAD SPECTRUM CLOCK GENERATOR, which isincorporated by reference herein.

When designing an SSC profile to notch out frequency bands of interest,different factors may be considered. For example, upper harmonic notcheswill be wider than the notch from a clock's fundamental frequency orother lower harmonics. So, for example, if a 100 MHz. clock is to bespread, and the notch is to avoid problematic harmonics over a 10 MHzband centered at 2.5 GHz, only a 0.4 MHz. notch (99.8 to 100.2 MHz) isACTUALLY required to attain a 10 M Hz. notch (2.495 to 2.505 GHz) in theRFI energy spectrum region of interest where the 2.5 GHz harmonic wouldotherwise be located. Other considerations may be made. In someembodiments, the notch width (corresponding to height of verticalcomponents in the utilized profile) should be weighed against EMIcompliance considerations. that is, the energy must go somewhere. Widernotches will typically result in higher energy contours surrounding thenotch. Along these lines, it may be desirable to make the profilesymmetrical above and below the notch region of the profile, resultingin a more symmetrical energy spread on either side of the spectralnotch. This may also make it easier to achieve EMI compliance. Notealso, that while sharp notches may be desired, they may not be as easyto practically implement, because, for example, limitations inattainable SSC profile verticality and PLL/DLL (phase locked loop/delaylocked loop) tracking limitations may make such sharp notchesprohibitive.

FIG. 3 is a block diagram of a portion of a computing platform 300implementing principles of the invention. The depicted diagramillustrates the platform 300 comprising a processing unit 302,controllable clock generators (a.k.a. clock sources or clocks) 312, andradios 314. The processing unit 302 may be part of any suitableprocessing logic, e.g., part of a processor core, System-on-chip,controller, dedicated logic, etc. It comprises RFI mitigation logic 304to control the spread spectrum clocking (SSC), as needed or desired, ofthe clocks 312 to mitigate against problematic RFI on radios 314.

The clocks 312 may be part of or used for various different devices inthe platform. For example, they could be used for memory interfaces,display clocking, peripheral device interfaces such as for USBinterfaces, PCIe interfaces, storage drive (e.g., SATA) interfaces, andthe like. In some cases, previous solutions such as shifting the clockfrequencies to avoid sensitive RFI regions may not be feesible sincemany clocks require extremely tight frequency deviation tolerances. Forexample, DATA and DVI interface specifications can require less than0.5% deviation from a fixed, predefined fundamental frequency.

The radios 314 correspond to the one or more various radios that may bepart of a platform. (Note that the term “platform” is intended toencompass any computing platform, portable or not portable, that cantake advantage of the RFI mitigation principles presented herein.Examples of platforms include but are not limited to cellulartelephones, tablets, netbook computers, notebook computers, internettelevision devices, MP3 players, some personal computers, and the like.)A platform may have one or more radios for providing it with differentservices including but not limited to wireless packet data networkconnectivity (e.g., WiFi), GPS, cellular network connectivity (e.g.,LTE, WiMax, GSM, etc.), and the like.

FIG. 4 is a flow diagram showing a routine that could be performed byRFI mitigation logic 304 in accordance with some embodiments.Essentially, at 404, it waits for a radio status change (402) from oneof the radios 314. Such a change could be a change in activity state(e.g., on, off, idle, etc.), or it could indicate that the radio ischanging its active channel. (Note that in alternative embodiments, itcould proactively check for changes or check and/or wait for changes.)At 406, it determines if the radio will be impacted by problematic RFIfrom a clock 312 with controllable SSC capability on the platform. Ifso, then at 408, if appropriate, it changes the clock(s)' SSC profile sothat it generates a notched energy spectrum so that the clock RFI doesnot harmfully impact the radio. At the same time, if possible, it shouldenable the clock(s) to comply with its required frequency performanceand EMI limitation requirements. (Note that not every clock may bechanged in this manner. For example, it might be decided that theclock's frequency tolerance is wide enough that the clock frequency canbe shifted instead of notching its energy spectrum.) In deciding whethera clock is problematic, the logic 302 may calculate or otherwise look-upthe possible energy created for the various harmonics of a clock to seeif the clock will impact the radio channel. If at 406, it was determinedthat there is not an SSC clock that should be changed, then the routineloops back to 404 and waits for another radio status change.

FIG. 5 is a diagram of a computing platform with RFI mitigation in aportable computer implementation. The depicted platform comprises a CPUchip 502 coupled to a platform control hub (PCH) 530 via a direct mediainterconnect (DMI) interface 514/532. The platform also includes memory511 coupled through a memory controller 510 and a display 513 coupledthrough a display controller 512. It also includes a storage drive 539(e.g., a solid state drive) coupled through a drive controller such asthe depicted SATA controller 538 and devices 518 (e.g., networkinterface, WiFi interface, printer, camera, cellular network interface,etc.) coupled through platform interfaces such as PCI Express (516 inthe CPU chip and 546 15401 in the PCR chip) and USB interfaces 536, 544.

The CPU chip 502 comprises one or more processor cores 504, a graphicsprocessor 506, and last level cache (LLC) 508. One or more of the cores504 execute operating system software (OS space) 520, which comprisesradio APIs (application platform interfaces) 522 and an RFI mitigationprogram 524. The radio APIs 522 each monitor or link to a separate radio(e.g., in a WiFi, cellular, or GPS device 518), among other reasons, todetermine if their status has changed and to report the status change tothe RFI mitigation program 524. It should also convey to the RFImitigation program the particular channel information for its associatedradio. The RFI mitigation program 524 then performs a routine such asthe one shown in FIG. 4 to control, via SSC notching, any problematicclocks in the platform to generate energy with notches to avoidunreasonably impairing a radio. (Note that while in this embodiment, theRFI mitigation logic 524 is implemented with software in the OS, itcould be implemented in any suitable manner. For example, it could beimplemented in firmware, either on the CPU or PCH chip.)

In the preceding description and following claims, the following termsshould be construed as follows: The terms “coupled” and “connected,”along with their derivatives, may be used. It should be understood thatthese terms are not intended as synonyms for each other. Rather, inparticular embodiments, “connected” is used to indicate that two or moreelements are in direct physical or electrical contact with each other.“Coupled” is used to indicate that two or more elements co-operate orinteract with each other, but they may or may not be in direct physicalor electrical contact.

It should also be appreciated that in some of the drawings, signalconductor lines are represented with lines. Some may be thicker, toindicate more constituent signal paths, have a number label, to indicatea number of constituent signal paths, and/or have arrows at one or moreends, to indicate primary information flow direction. This, however,should not be construed in a limiting manner. Rather, such added detailmay be used in connection with one or more exemplary embodiments tofacilitate easier understanding of a diagram. Any represented signallines, whether or not having additional information, may actuallycomprise one or more signals that may travel in multiple directions andmay be implemented with any suitable type of signal scheme, e.g.,digital or analog lines implemented with differential pairs, opticalfiber lines, and/or single-ended lines.

It should be appreciated that example sizes/models/values/ranges mayhave been given, although the present invention is not limited to thesame. As manufacturing techniques (e.g., photolithography) mature overtime, it is expected that devices of smaller size could be manufactured.In addition, well known power/ground connections to IC chips and othercomponents may or may not be shown within the FIGS, for simplicity ofillustration and discussion, and so as not to obscure the invention.Further, arrangements may be shown in block diagram form in order toavoid obscuring the invention, and also in view of the fact thatspecifics with respect to implementation of such block diagramarrangements are highly dependent upon the platform within which thepresent invention is to be implemented, i.e., such specifics should bewell within purview of one skilled in the art. Where specific details(e.g., circuits) are set forth in order to describe example embodimentsof the invention, it should be apparent to one skilled in the art thatthe invention can be practiced without, or with variation of, thesespecific details. The description is thus to be regarded as illustrativeinstead of limiting.

What is claimed is:
 1. An apparatus, comprising: a radio; a spreadspectrum clock (SSC) having an associated SSC profile; and mitigationlogic to control the profile to generate a notch in a radio frequencyinterference (RFI) energy spectrum, the notch to align with a wirelesschannel to be used by the radio, wherein the notch is to mitigateinterference for the wireless channel, and wherein a width associatedwith the notch is based, at least in part, on at least oneelectromagnetic interference (EMI) compliance characteristic.
 2. Theapparatus of claim 1, wherein the radio is part of a wireless interface.3. The apparatus of claim 1, wherein the radio is part of a globalpositioning system (GPS) module.
 4. The apparatus of claim 1, whereinthe SSC is spreadable through discrete spread spectrum clocking.
 5. Theapparatus of claim 4, wherein a triangular discrete spread spectrumclocking (DSSC) profile with a vertical gap corresponding to the channelto be used by the radio is employed.
 6. The apparatus of claim 4,wherein a non-triangular DSSC profile with a vertical gap correspondingto the channel to be used by the radio is employed.
 7. The apparatus ofclaim 1, wherein the mitigation logic is implemented with operatingsystem software.
 8. The apparatus of claim 1, wherein the mitigationlogic is implemented with firmware.
 9. A computing platform, comprising:mitigation logic; devices with radios, each device to operate in atleast one channel to be reported to the mitigation logic; and clocksthat are to be spread using discrete spread spectrum clocking (DSSC),the DSSC for each clock to be controlled via a DSSC profile, themitigation logic to select a profile for a clock that results in anotched spectrum to mitigate interference with the at least one channel,and wherein the notched spectrum includes a plurality of notches thatare based, at least in part, on at least one electromagneticinterference (EMI) compliance characteristic, and wherein triangularprofiles with vertically displaced gaps are used to create the pluralityof notches.
 10. The computing platform of claim 9, comprising aprocessor chip to execute an operating system that incorporates themitigation logic.
 11. The computing platform of claim 10, in which theoperating system incorporates application program interfaces (APIs) toreport status changes including channel usage for the device radios. 12.The computing platform of claim 9, comprising a chip including firmwareto implement the mitigation logic.
 13. The computing platform of claim9, in which the mitigation logic is in a separate DSSC module for eachclock.
 14. A method, comprising means for: determining that a radio in acomputing platform is to receive information over a wireless channel;determining whether a clock in the platform will generate problematicradio frequency interference (RFI) in the wireless channel; and causingspread spectrum clocking (SSC) to be performed on the clock, the SSCusing a profile that will result in an RFI spectrum with a notch in atleast part of the channel region, wherein the notch is to mitigateinterference for the wireless channel, and wherein a width associatedwith the notch is based, at least in part, on at least oneelectromagnetic interference (EMI) compliance characteristic.
 15. Themethod of claim 14, in which the method is performed by RFI logic in acomputing platform.
 16. The method of claim 14, in which the SSC isdiscrete SSC.
 17. The method of claim 16, in which a triangular profilewith a vertically displaced gap is used.